Nonlinear Uniaxial Material Model for Reinforcing Steel Bars

Recent advances in nonlinear analysis of reinforced concrete structures have been based primarily on fiber-based discretization of the member cross section at locations of expected inelastic action. However, the accuracy of a fiber-section model is almost entirely dependent on the ability of constitutive material models to represent the overall inelastic behavior of the member. The postyield response of an RC element is controlled by the longitudinal reinforcing steel and the confinement provided by the transverse reinforcement. While adequately confined concrete exhibits stable hysteretic behavior, poorly confined sections degrade rapidly initiated in part by buckling of the longitudinal reinforcement. Particular attention must, therefore, be given to the stress-strain model used to represent the behavior of reinforcing steel bars. The development and validation of an advanced material model for reinforcing bars in RC members is presented in this paper. First, a base material model to describe the primary cyclic stress-strain relationship of reinforcing steel is developed. Using available concepts to initiate bar buckling, low-cycle fatigue fracture, and cyclic degradation, a generic phenomenological material model is developed and implemented in an open-source computational platform. The effectiveness of the new reinforcing bar model is validated through comparison with available experimental data. DOI: 10.1061/ ASCE 0733-9445 2009 135:4 335 CE Database subject headings: Buckling; Degradation; Fatigue; Constitutive models; Nonlinear analysis; Reinforced concrete; Bars.